1. Field of the Invention
The present invention relates to thermographic recording films, and more specifically, it relates to the use of a crosslinking compound containing at least two epoxide moieties in a protective layer and/or in a layer on top of the protective layer of certain thermographic recording films which are to be imaged with a thermal printhead. The crosslinking compound helps to prevent gouging, to reduce head build-up on the thermal printhead, enhance print performance and to improve the image quality of the printed image.
2. Description of the Related Art
There are disclosed in the art a number of image-forming systems for use in thermographic recording films. One of these image-forming systems utilizes color-forming di- and triarylmethane compounds possessing certain S-containing ring closing moieties, namely a thiolactone, dithiolactone or thioether ring closing moiety as are disclosed in European Pat. No. 250,558 and U.S. Pat. No. 5,196,297 of E. J. Dombrowski, Jr. et al. These dye precursors undergo coloration by contacting with Lewis acid material, preferably a metal ion of a heavy metal, particularly silver, capable of opening the S-containing ring moiety to form a colored metal-complex.
As disclosed in the above-cited patents, the ability of these dye precursors to form a colored dye almost instantaneously when contacted with Ag+ renders them eminently suitable for use as color formers in thermal imaging systems employing organic silver salts, such as silver behenate. These thermographic recording films preferably include a heat-fusible organic acid material. U.S. Pat. No. 4,904,572 of E. J. Dombrowski, Jr. et al, issued Feb. 27, 1990, discloses 3,5-dihydroxybenzoic acid as a preferred heat-fusible organic acid.
The above described thermal color-forming system preferably employs a thermoplastic binder, e.g. polyvinylbutyral. When imagewise heating is accomplished by means of a thermal printhead, the thermoplastic binder is in direct contact with the thermal printhead during imaging. Since thermoplastic binders soften upon the application of heat, they tend to stick to the thermal printhead during imaging. This "sticking" interferes with the printing, adversely affects image quality, and can cause damage to the printhead.
A number of ways to prevent sticking between a binder and a thermal printhead during printing have been suggested for various thermographic recording films. Many of these employ a protective or anti-stick topcoat comprising silica over the thermographic color-forming layer. These topcoats contact the thermal printhead during imaging to prevent "sticking". Another way to prevent sticking has been to employ a surface active agent to add anti-stick properties. However, these silica containing topcoats and surface-active agents have drawbacks and/or do not perform adequately when the binder employed in the coloring system is polyvinylbutyral and the support used for the thermosensitive recording film is a transparent support.
For example, low surface energy materials such as silicone polymers exhibit good anti-stick properties. However, the useful silicone polymers are relatively low molecular weight silicone polymers which have a tendency to be migratory and thus cause problems, e.g., they transfer to the back of the film if it is rolled for storage or to the back of the adjacent film if stored in sheets. In addition, because these silicones are polymers, their properties change with changes in moisture and temperature and therefore, their performance is not consistent under all conditions.
U.S. Pat. No. 4,583,103 issued Apr. 15, 1986 and U.S. Pat. No. 4,820,682 issued Apr. 11, 1989 disclose protective topcoats for heat-sensitive recording papers containing a binder comprising silicon modified polyvinylalcohol and colloidal silica and/or amorphous silica. The above patents also disclose topcoats wherein said colloidal silica contains silica grains having an average particle size of from about 10 millimicrons (m.mu.) to 100 m.mu.(1 m.mu.=1 nanometer (nm)) and the amorphous silica has primary grain size of about 10 micrometers (.mu.m) to 30 .mu.m (1 .mu.m =10.sup.3 nm). These topcoats are disclosed as providing good printing densities, resistance to various chemicals, oils and water, and anti-sticking and anti-blocking properties. In addition, the latter patent discloses the topcoat as exhibiting excellent transparency and describes it for use on a transparent base. However, the lowest level of haze reported is 16%, a level which is higher than desirable for overhead transparency (OHT) applications.
Published UK Patent Application No. 2,210,702 having a publication date of Jun. 14, 1989 and assigned to the same assignee as the latter two patents, discloses a heat-sensitive recording material which, when it employs a topcoat as described above, e.g., silicon modified polyvinylalcohol and colloidal silica, reports a level of haze as low as 8%.
However, when polyvinylbutyral is used as the binder for the color-forming materials of this invention, and a topcoat as described above, i.e. silicon modified polyvinylalcohol and colloidal silica, is employed to prevent sticking, there is poor adhesion between the topcoat and underlying polyvinylbutyral layer, as well as poor scratch resistance of the resulting film. In addition, the silicon modified polyvinyl alcohol binder is water soluble and can be rubbed off with water.
U.S. Pat. No. 4,985,394 issued Jan. 15, 1991 discloses a topcoat for a thermosensitive recording material which comprises at least one inorganic pigment selected from the group consisting of silica and calcium carbonate, each having an average particle diameter of 0.1 .mu.m or less, and a water-soluble binder, formed on the thermosensitive coloring layer. Many of these topcoats have problems of inadequate transparency and/or adhesion when coated over the polyvinylbutyral color-forming layer of the present invention.
U.S. Pat. No. 5,198,406 of J. M. Mack and K. Sun, assigned to the assignee of the present application, discloses a topcoat for transparent thermographic recording films using the above color-forming system. Specifically, the transparent thermographic recording films described therein comprise a transparent support carrying:
(a) a dye image-forming system comprising a di- or triarylmethane thiolactone dye precursor, an organic silver salt, a heat-fusible organic acidic material, and polyvinylbutyral as the binder; and, PA1 (b) a protective topcoat layer positioned above said dye image-forming system and comprising a water-insoluble polymeric binder, a mixture of at least two colloidal silicas having different average particle diameters in the proportion, by weight, of 1 part of silica having an average diameter of 50 nm or smaller and 0.3 to 1 part of silica particles having an average diameter no more than 40% of the larger sized silica particles, the ratio of total silica to binder being at least 3 parts per weight silica to 1 part per weight binder. PA1 (a) an image-forming system; and, PA1 (b) a protective layer comprising colloidal silica. The thermographic recording film additionally includes a multiepoxy compound in the protective layer and/or in a layer on top of said protective layer. The ratio (by weight) of colloidal silica to said multiepoxy compound is at least 2:1, and preferably in the range of from 2:1 to 15:1; a particularly preferred range is from 2.5:1 to 5:1. At ratios of less than 2:1 there is too little silica present so that sticking may occur. However, at ratios exceeding about 15:1 the integrity of the film tends to be compromised, e.g., crazing and/or cracking of the film may occur.
While the above described topcoat prevents sticking of the polyvinylbutyral color-forming layer(s) to the thermal printhead during printing, with certain high energy thermal printers, e.g. Model BX 500 high density printer, commercially available from Seikosha America, Inc., Mahwah, N.J. and Model TDU 850 commercially available from Raytheon Company, Submarine Signal Division, Portsmouth, R.I., there are the problems of gouging on the surface of the recording film and head build-up on the thermal printer.
"Gouging" results in actual depressions or indentations in the recording film which can be either continuous or intermittent. Gouging is believed to be caused by high temperatures, pressure and/or sticking.
"Head build-up" is the build-up of components of the thermographic recording film on the thermal printhead. Head build-up can cause streaking in the printed image, decreased image density with continued printing and damage to the thermal printhead. Head build-up can become so pronounced, particularly when a lubricant, e.g. polytetrafluoroethylene, is present in the topcoat, that it appears as "spiderwebs" on the thermal printer.
"Streaking" is believed to be the result of the insulating effect of head build-up on the printing element(s) of the thermal printhead which interferes with printing causing linear discoloration ("streaking") in the printed image.
The presence of a lubricant in the topcoat is generally desired to impart slip characteristics and to decrease gouging of the printed image, however, head build-up usually becomes more pronounced when a lubricant, e.g. polytetrafluoroethylene, is used in the topcoat. Generally, the greater the concentration of lubricant, the greater the degree of head build-up.
The aforementioned U.S. Pat. No. 5,198,406 of J. M. Mack et al., discloses the use of organofunctional silanes in the topcoat or in a layer on top of the topcoat to react with both the silica and the binder(s) in the topcoat thereby functioning as a coupling agent to join the two and thereby reinforce and strengthen the silica/polymeric binder matrix. The addition of the organofunctional silane helps to reduce head build-up and improves the scratch resistance of the recorded image.